From Bulbapedia, the community-driven Pokémon encyclopedia.
Unlike other Pokémon games, third version games rarely contain prototype information because they are based on an already existing game. Pokémon Emerald is no exception.
Very little was changed in relation to Pokémon Ruby and Sapphire, with the exception of the Battle Frontier and storyline tweaks to better include the legendary Pokémon Kyogre, Groudon, and Rayquaza. While there is no evidence to suggest the storyline was changed seriously, there is evidence that the Battle Frontier was changed slightly in various stages of development; however, these changes are minor and strictly relate almost entirely to building placements and static objects, such as fences.
In addition to the tracks present in Ruby and Sapphire (including the upconverted music from Generation II), Emerald contains the tracks from Pokémon FireRed and LeafGreen alongside its own.
A new, empty header included after the empty Kanto and Sevii Islands ones is named "Special Area". There is no actual information on what would have been included in this area, so it might have been intended merely to be a name for an event. This is probably the same case with the map header named 'SPECIAL' in Generation II with index number 0. However, it is read as Celadon Dept. in FireRed and LeafGreen.